20141002020853_create_birds.rb

class CreateBirds < ActiveRecord::Migration
  def change
    create_table :comments do |t|
      t.string :location
      t.belongs_to :birdable, polymorphic: true

      t.timestamps
    end
  end
end

bag.rb

class Bag < ActiveRecord::Base
  
  def put_a_bird_on(location)
    birds.create(location: location)
  end
end

bird.rb

class Bird < ActiveRecord::Base
  belongs_to :birdable, polymorphic: true
end

usage.rb

bag = Bag.new
bag.put_a_bird_on("it")